The present invention relates in general to floating roof tanks, and, more particularly, to drainage systems for such floating roof tanks.
In floating roof tanks, that is those tanks having no fixed roof, any water which collects on the roof, for example through precipitation falling on the roof, should not be drained into the product stored in the tank. Therefore, drainage systems for floating roofs generally include some type of drain line, such as a pipe or a hose, fluidly connecting a drain point on the floating roof to a drain point outside the tank, with such drain line passing through a wall of the tank. In such a system, the water from the roof usually enters the drain line via a sump located at the center of the floating roof, then drains through the line and exits the tank through a valve located near the bottom of the tank wall.
Historically, these drain lines are subject to many drawbacks. For example, because the drain line passes through the stored product, and as spillage of the product is undesirable, the drain valve to which the drain line is connected is usually kept closed. Thus, water is allowed to accumulate on the floating roof until an operator decides that the roof should be drained. The valve is then manually opened to drain the water from the floating roof. However, if a rupture in the drain line occurs while the manual valve is open, product may escape from the tank via the drain valve. Such product escape is highly undesirable as, not only is valuable product lost, but safety hazards are created.
Floating roof drainage systems have heretofore been of two basic designs. The first design includes a hose drain, and in this design, a hose is attached to a sump on the floating roof, runs through the product, and is then attached to a penetration in the tank wall just upstream of the drain valve. This hose must be weighted because it is normally dry and self-buoyant. Such hoses are generally made of reinforced rubber-like materials, and are subject to machanical and chemical abuse from the operation of the tank and/or from the product stored in the tank.
A second design has included pipes interconnected by swing joints. The concept of this second design is to provide a conduit which is more resistant to mechanical and/or chemical abuse than are the hoses in the first design. However, the swing joint design also has weaknesses, and the primary weakness of the swing joint design appears in the joints and seals used in those joints. These swing joint designs often leak product into the drainage system.
When weighted, the hose rests on the tank bottom. In this position the hose can be frozen into any water that collects below the product in the tank. Subsequently, upward movements of the floating roof can damage the hose. Also, the hose cannot drain completely dry since the hose on the tank bottom is lower than the attachment thereof to the drain penetration on the tank shell. This trapped water can freeze with resultant damage to the hose.
Other drawbacks to heretofore known drainage systems include buoyance induced looping of the hoses, and tangling, kinking and crushing of the hoses if the hoses float freely in the tank.
The inventor is also aware of drainage systems which include articulated drain pipes. An example of such articulate systems is disclosed in U.S. Pat. No. 2,717,095 issued to M. W. Gable. In the Gable patent, a plurality of rigid drain pipes are connected together by compound joints which each has a structural hinge and an independent flexible liquid connection. However, the joints in this drainage system are still subject to failure and suffer drawbacks similar to those already discussed.
The inventor is also aware of a drainage system which includes a loop of steel pipes interconnected by bolted flange-type joints at the bends in the loop. The bolted connections in this system do not flex one relative to the other, but remain fixed. As the floating roof moves, a complicated system of cables picks up the loop and extends the dimensions of that loop in a vertical direction. Such an extension stresses the pipe loop, and this stressing is minimized by prestressing the loop as it is installed. As the tank is worked and the floating roof moves, the prestress is reduced to zero, and eventually the stress level becomes the negative of the prestress level.
This last-mentioned drainage system is subject to several drawbacks, however. The major drawback arises because of the need for gasketing between the flanges of the bolted joints of the system. Such bolted connections are subject to leaks, and any gasketing is thus subject to chemical attack by the stored product. Furthermore, the loop in this design is nearly constantly stressed throughout the entire length thereof. Thus, this design includes a system of pulleys and cables to pick up the entire loop when the floating roof is moved. These pulleys and cables are attached to the loop at a midpoint on that loop, and this point moves half travel as the floating roof moves. As the floating roof moves from the tank empty to the tank full position, this attachment point moves half that vertical distance. This design requires housings to protect the cable and pulley mechanism, and may not be usable in cold climates. The present invention replaces this system of cables and pulleys attached at one midpoint by a system of chains or the like attached at several locations along the length of the loop.
The inventor is also aware of the following patents which disclose drainage systems for floating roofs:
U.S. Pat. No. 2,315,023
U.S. Pat. No. 2,657,821
U.S. Pat. No. 2,482,468
German Pat. No. 236,427--1911